One Source Metal Products
Canadian store...don't know if they will ship to the US, and their products aren't broken down more than general categories. Too late to call today. Discount Steel - The widest selection of steel, aluminum and other metals at discount prices.
List 1045 in their barstock section, but don't break it down. Too late to call today. Impact Metal Products - Steel, high yield strength, low temp charpys & abrasion resistant flats
Has 1045 rounds, and says to call and ask about other shapes. Too late to call.
Tried a few other places on the phone, but none had 1045 flat bar.